This invention relates to a magnetically biased velocity change sensor of the type comprising a tube having a circular cylindrical inside diameter containing a magnetically permeable sphere whose diameter is just slightly less than the tube's inside diameter so that travel of said sphere within said tube is damped by gaseous fluid present within said tube, a magnet for biasing said sphere axially within said tube against a stop proximate one axial end of said tube, a switch disposed proximate the opposite axial end of said tube for actuation by said sphere in response to said sensor experiencing a velocity change of sufficient magnitude and duration to cause said sphere to overcome the bias of said magnet and travel within said tube from the position against said stop to a position causing said switch to give a signal indicative of the occurrence of such velocity change. Sensors of this type are presently in use in the automotive industry in passive occupant restraint (air bag) systems.
An example of this type of velocity change sensor is disclosed in U.S. Pat. No. to Breed 4,329,549 issued May 11, 1982. The mass production of this sensor requires very close control of part dimensions and tolerances in order to comply with functional specifications. Such need for this degree of control is a factor in the cost which the customer must pay for the sensor. If the cost of manufacturing the sensor can be reduced while compliance with applicable specifications is maintained, a meaningful savings can accrue. It is toward this objective that the present invention is directed.
One of the problems confronting a simplification in manufacturing procedures is the fact that the response characteristic of a particular sensor for a particular automotive vehicle model is usually unique. In other words, the sensor manufacturer must build a number of different models of sensor, even though the models are basically the same. One aspect of the present invention involves an improvement whereby certain models of sensors differ in their response characteristics solely by differences in the viscosities of their gaseous damping fluids. This improvement enables the sensors to share component parts of identical dimensions and tolerances, thereby simplifying manufacturing considerations.
Another aspect of the invention involves a reduction in the strictness of dimensional tolerance requirements for certain component parts, in particular the diameter of the sphere and the inside diameter of the tube. This improvement is accomplished by a novel construction for the sphere which retains its magnetic permeability, but with a significantly lower mass. Exactly how this occurs will be seen in the detailed description of a preferred embodiment. A drawing also accompanies the description. Taken together, the description and drawing present the best mode contemplated by the inventor at the present time for carrying out the invention. Additional advantages, features, and benefits of the invention will suggest themselves to the reader as the disclosure proceeds.